Solid-state batteries were previously thought to store and release charge by nudging ions back and forth between two electrodes so that the ions flow through the battery’s solid electrolyte in a gentle stream. That seems to be wrong.
Viewed on an atomic scale, the smooth flow is an illusion, and individual ions hop erratically from one open space to another within the roomy atomic lattice, nudged in the direction of an electrode via a steady voltage. The hops are difficult to predict and a challenge to trigger and detect.
In a new study, researchers gave ions a voltage jolt by hitting them with a pulse of laser light. Most of the ions briefly reversed direction and returned to their previous positions before resuming their usual, random travels. The ions “remembered” where they had just been.
The team from the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University, Oxford University, and Newcastle University published their findings in Nature.
“You can think of the ions as behaving like a mixture of cornstarch and water,” said Andrey D. Poletayev, a postdoctoral researcher at Oxford who helped lead the experiment when he was a postdoc at SLAC. “If we gently push this cornstarch mixture, it yields like a liquid; but if we punch it, it turns solid. Ions in a battery are like electronic cornstarch. They resist a hard shake from a jolt of laser light by moving backwards.”
The ions’ “fuzzy memory,” lasts only a few billionths of a second. Now, however, scientists can predict, for the first time, what traveling ions will do next – an important consideration for new materials.
Until now, the way the ions travel was believed to be a classic “random walk:” They jostle, collide, and bumble along, like a drunk person staggering down a sidewalk, but eventually reach some destination in a way that can seem deliberate to an observer. Or think of a skunk releasing stinky spray into a room full of people; the molecules in the spray randomly jostle and collide but all too quickly reach your nose. That turned out to be wrong. Until now, previous researchers haven’t been able to observe what was seen in this study.
The atomic-scale discoveries will help bridge the gap between the atomic motions that can be modeled in a computer and a material’s macroscopic performance—making the research complicated.
